Update EphemeralMongo to fix flaky tests with "Text file busy" on Ubuntu

[bkoelman]

Related to asimmon/ephemeral-mongo#91.

QUALITY CHECKLIST

• Changes implemented in code
• Complies with our contributing guidelines
• N/A: Adapted tests
• N/A: Documentation updated

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 94.77%. Comparing base (663f215) to head (d76c31f).
Report is 1 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master      #97   +/-   ##
=======================================
  Coverage   94.77%   94.77%           
=======================================
  Files          13       13           
  Lines         249      249           
  Branches       37       37           
=======================================
  Hits          236      236           
  Misses          7        7           
  Partials        6        6           

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